This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Fe K-edge XAS data will be measured on the electron transfer protein cytochrome c (cytc). The c-type cytochromes that have histidine-methionine (His-Met) iron coordination play important roles in electron-transfer reactions and in enzymes. The axial methionine ligand plays a major role in stabilizing the protein and maintaining a low redox potential. Protein mutations of Pseudomonas aeruginosa (Pa) and Nitrosomonas europeae (Ne) cytc show differences in rhombicity (using EPR spectroscopy) in their active site, which have been linked to perturbations in the Fe-S(Met) bond. Fe K-edge EXAFS data (BL9-3) will shed light on specific geometric structure changes at the electron-transfer active site with changes in the protein environment. The experiments will be extended to S K-edge XAS (BL4-3) to obtain quantitative information on the Fe-S covalency.